// 大屏
@mixin lg {
    @media only screen and (max-width: 1200px) {
        @content;
    }
}
// 桌面
@mixin md {
    @media only screen and (max-width: 992px) {
        @content;
    }
}
// 平板
@mixin sm {
    @media only screen and (min-width: 768px) {
        @content;
    }
}
// 手机
@mixin xs {
    @media only screen and (min-width: 480px) {
        @content;
    }
}
